1. Field of the Invention
This invention relates to an energy monitoring system for the supervision of a particle accelerator, preferably of a linear accelerator. Particularly, this invention relates to an electron energy interlock system for an electron linear accelerator of the type having no electron beam bending system which may act as an electron energy band pass.
2. Description of the Prior Art
It is known in the art of radiation systems of various types to switch off the radiation beam via an ionization chamber to which the radiation is applied, as soon as a previously determined dosage of radiation has been reached. Particularly in the case of particle accelerators, such as linear accelerators, it is known to use monitoring systems which control dosage and dosage rate during treatment and which ensure automatic termination of radiation if preset values are exceeded (see brochure "Mevatron 20" by Siemens AG, West Germany, Order No. MT 3/1702.101-WS 5791, particularly see page 9). Such safety interlock systems may be applied in linear accelerators in which the dose rate is uniformly fixed for X-ray and electron irradiation of all energies, such as to a value of 300 rad/min in the flattened field at 100 cm FD (see brochure "Mevatron 20", supra), or in linear accelerators in which the dose rate is continuously variable between a lower and an upper limit (see brochure "Mevatron 60, Data" by Siemens AG, West Germany, Order No. MT 3 -6027.101-PA 9783).
U.S. Pat. No. 4,115,830 discloses a monitoring system. for the high voltage supply of an ionization chamber. This system is preferably used for monitoring a particle accelerator. In the field of particle accelerators, it is known to regulate the radiation intensity or radiation output via the ionization current of an ionization chamber subjected to the radiation in such a way that the number of radiation pulses per time unit is changed in correspondence with the chamber signal measured. To overcome inaccuracies in the ionization current measurement below a minimum value of the high voltage supplied to the chamber, the monitoring system is provided. The monitoring system comprises a switch member which is associated with a safety circuit of the particle accelerator and which switches off the latter in the event of insufficient high voltage.
It is desirable to provide another interlock system for a particle accelerator, namely an energy interlock system that interlocks the accelerator in case of undesired energy changes of the radiation output. Such an energy interlock system for the accelerated electrons and/or X-rays is especially important in a linear accelerator which does not dispose of an electron beam bending system (see, for instance, brochure "Mevatron 60", supra). Such an electron bending system, usually a bending magnet system, commonly works as an energy filter or band pass for accelerated electrons (see, for enstance, brochure "Mevatron 20", supra). A linear accelerator of the type having no electron beam bending system may experience a drift of signals from its mechanical and electrical components which leads to an electron output energy that is too high or too low for the intended irradiation process. Even though a dose monitoring system and a dose rate monitoring system may be working properly, a patient who is irradiated by the accelerator should be protected from too high or too low electron or X-ray energies.
Assume, for instance, that a linear accelerator disposes of a dose rate control or servo circuit. If for some reason (for instance: drift of components or source variations) the beam current which may be measured by an ionization chamber indicates an increase, while the radio frequency supplied by the HF source of the accelerator remains unchanged, the energy of the accelerated electrons and/or X-rays will increase. Such an increase in energy has to be avoided, as soon as a preset maximum energy value is reached. However, if by some reasons (drift of components) the radio frequency power supplied by the HF source of the accelerator should increase, while the output dose rate (in r/min) is kept constant by the dose rate control circuit, the energy of the accelerated electrons and/or X-rays would also increase. Such energy increase has to be stopped, as soon as the preset maximum energy level is reached. The same applies to energies which are too low. A decrease in energy should be stopped, as soon as a preset minimum energy level is reached.